1. Field of the Invention
The present invention relates to an optical beam scanning system comprising a rotary polygon mirror for deflecting a light beam from a light source and thereby scanning a surface-to-be-scanned by the deflected light beam.
2. Description of the Prior Art
FIGS. 1A and B are respectively a schematic elevational view and a schematic plan view of a prior art optical beam scanning system. In the optical beam scanning system, a light beam L from a light source (not shown) is deflected by mirror surfaces 1aof a rotary polygon mirror 1 which rotates in the direction of the arrow A of FIG. 1B. The deflected light beam L.sub.2 is focused on a surface 3 to be scanned through a scanning lens 2. Thus, a beam spot SP which is formed on the surface 3, moves along a scanning direction y at a prescribed speed in accordance with the rotational speed of the rotary polygon mirror 1.
In FIG. 1A is shown a beam spot SP' which is deviated in a direction x substantially perpendicular to the scanning direction y. Assuming an angle of deviation or tilt of .phi. from the axis of rotation for the rotary polygon mirror 1, the position of the beam spot SP' in the direction x, i.e. facet error, will be displaced from the desired position SP of FIG. 1A by an amount .DELTA. proportional to the angular deviation 2.phi. of the light beam L.sub.2. The term "facet error" refers to the deviation of the beam spot of the light beam L.sub.2 in the direction x. In such an optical beam scanning system, it is required that the rotary polygon mirror 1 be manufactured such that each mirror surface 1a is completely in parallel with the axis of rotation of the mirror. To produce a high-accuracy rotary polygon mirror has a limitation, however, such that the respective mirror surfaces 1a of the polygon mirror 1 are not completely parallel but instead, are slightly inclined with respect to the axis of rotation at dispersed angles. Thus, facet error occurs in the optical beam scanning system of FIG. 1 and may causes poor results in terms of the quality of image transfer to the surface 3.
In order to solve this problem, the mirror surfaces 1a of the rotary polygon mirror 1 are made optically conjugate with the surface 3 with respect to a direction normal to the scanning plane (equivalent to the yz-plane in FIG. 1). In more concrete terms, this is accomplished by disposing a cylindrical lens 4 having refracting power only along the direction x between the scanning lens 2 and the surface 3, as shown in FIGS. 2A and 2B. Further, as shown in FIGS. 2B and 2C, a first image-forming optical system 5 is disposed between the light source (not shown) and the rotary polygon mirror 1. This forms a light beam L.sub.1 which is of parallel rays along the direction y and which is focused on the mirror surfaces 1a of the rotary polygon mirror 1 along the direction x. Consequently, the mirror surfaces 1a of the rotary polygon mirror 1 are optically conjugate with the surface 3, whereby the facet error is compensated.
Although facet errors are overcome in the optical beam scanning system, including the first image-forming optical system 5, the following problem takes place: Namely, an image field is so curved that an image-forming position deviates from the surface 3 along an optical axis direction z; and the size of the beam spot SP change particularly when the amount of deviation of the beam spot is out of a range of focal depth for the optical beam scanning system.
In order to solve this problem, two techniques have been generally proposed.
(1) One of them is disclosed in Japanese Patent Laid-Open Gazette No. 58-200214 or 58-179813. According to the technique disclosed therein, as shown in FIG. 3 and more particularly FIG. 3B, a cylindrical lens 6 curves so as to approach a surface 3 to be scanned from its optical axis OA toward both end portions thereof, to thereby correct curvature of field.
When the optical beam scanning system includes the cylindrical lens 6 formed as shown in FIG. 3, however, the cylindrical lens 6 must be disposed in the vicinity of the surface 3. The reason for this is that it becomes difficult to correct the curvature of field only by disposing the curved cylindrical lens 6 between the scanning lens 2 and the surface 3 as the overall cylindrical lens 6 is separated from the surface 3. The further the lens 6 is separated from the surface 3, the greater its length is required to be to achieve a large effective scanning width and this results in difficulty in manufacturing. If the cylindrical lens 6 is arranged in the vicinity of the surface 3, further, the beam diameter of a light beam L.sub.3 passing through the cylindrical lens 6 is reduced. This results in imaging performance being reduced by dust or the like.
(2) Another technique is that described in Japanese Patent Laid-Open Gazette No. 62-265615, 61-275814 or 60-133416. The references disclose a method or correcting curvature of field by reducing refracting power of a cylindrical lens along a direction (equivalent to the above direction x) normal to a scanning plane (yz-plane) from its optical axis toward both ends thereof. As shown in FIG. 4, for example, a cylindrical lens 7 is so formed that a radius of curvature along a direction x is increased from an optical axis OA toward both ends of the lens 7, to change effective refracting power along the direction x as described above.
Although such lens design is possible in theory, an expression for a configuration of the lens obtained by the design is so complicated that it may be difficult to work a lens in accordance with the expression.